The present invention relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to an interdigitated enthalpy exchange device for a fuel cell power plant that exchanges heat and water exiting the plant back into the plant to enhance water balance and energy efficiency of the plant.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (xe2x80x9cPEMxe2x80x9d) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (xe2x80x9cPEMxe2x80x9d) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode including water resulting from proton drag through the PEM electrolyte and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
As fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks, buses, etc., maintaining a water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize fuel cell power plant weight and space requirements, the plant must be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset water losses from gaseous streams of reactant fluids passing through the plant. For example, any water exiting the plant through a cathode exhaust stream of gaseous oxidant or through an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode and retained within the plant.
An additional requirement for maintaining water self-sufficiency in fuel cell power plants is associated with components necessary to process hydrocarbon fuels, such as methane, natural gas, gasoline, methanol, diesel fuel, etc., into an appropriate reducing fluid that provides a hydrogen rich fluid to the anode electrode. Such fuel processing components of a fuel cell power plant typically include a boiler that generates steam; a steam line that directs the steam from the boiler; and a reformer that receives the steam and a hydrocarbon fuel mixture along with a small amount of a process oxidant such as air and transforms the mixture into a hydrogen-enriched reducing fluid appropriate for delivery to the anode electrode of the fuel cell. The fuel processing components include water requirements that are part of an overall water balance of the fuel cell power plant. For example, water made into steam in the boiler must be replaced by water recovered from the plant.
It is known to use a direct mass and heat transfer device to enhance water balance of a fuel cell power plant, such as disclosed in U.S. Pat. No. 6,007,931 that issued on Dec. 28, 1999 to Fuller et al., and is owned by the assignee of all rights in the interdigitated enthalpy exchange device invention disclosed herein, and which Patent is hereby incorporated herein by reference. The direct mass and heat transfer device passes the process oxidant stream upstream of the fuel cell in mass transfer relationship with the plant exhaust stream exiting the plant so that mass and heat such as water vapor and entrained liquid water in the exhaust stream pass directly through a mass transfer medium into the process oxidant stream to heat and humidify the oxidant stream without the complexities of traditional condensing heat exchangers. Therefore the Fuller et al. mass and heat recovery system substantially enhances water recovery and plant energy efficiency because the recovered water and heat need no parasite power from the fuel cell to pump or otherwise transfer the mass and heat to humidify and heat the process oxidant stream.
Another difficulty associated with fuel cell power plants utilized to power transportation vehicles involves a coolant system necessary to maintain the fuel cell within an appropriate heat range. Heat must be removed from the fuel cell, and it is common to cycle a cooling fluid through cooler plates adjacent reactant stream flow fields of the fuel cell. Such cooling fluids must also be tolerant of temperature extremes to which transportation vehicles are exposed, and therefore the cooling fluids consist of various antifreeze solutions as is well known. However, as a cooling fluid contacts cell components, especially where fuel processing components have supplied a reformate fuel to the fuel cell, it is also known that the cooling fluid will be contaminated with dissolved gases, such as ammonia (NH3), hydrogen (H2), as well as carbon dioxide (CO2). Additionally, it is known that dissolved metals and other ions must be removed in order to limit conductivity of the coolant fluid to avoid shunt current corrosion. Therefore it is known to use cooling fluid treatment systems within a fuel cell power plant, such as a degasifier to strip dissolved or dissociated contaminants within the cooling fluid, and a demineralizer to remove dissolved metals, such as shown in U.S. Pat. No. 4,344,850 to Grasso, which patent is also owned by the assignee of all rights in the interdigitated enthalpy exchange device invention disclosed herein.
It has more recently been discovered that the direct mass and heat transfer system and a coolant fluid treatment system may be functionally integrated as disclosed in U.S. patent application Ser. No. 09/544,103 entitled, xe2x80x9cFunctional Integration of Multiple Components For a Fuel Cell Power Plantxe2x80x9d, which U.S. Patent Application is commonly owned by the assignee of all rights in the interdigitated enthalpy exchange device invention disclosed herein, and which Patent Application is hereby incorporated herein by reference. As disclosed in that Application, a housing supports a mass transfer device in mass transfer relationship between the process oxidant stream entering the fuel cell and a plant exhaust stream exiting the plant, and the housing also includes a degasifier and a cooling fluid accumulator. Consequently, a separate cooling fluid degasifier is not necessary. The housing also receives heated gases from an anode exhaust burner associated with fuel processing components so that the mass transfer device also transfers water and heat from the burner exhaust gas. The degasifier may be any known mass transfer device capable of effecting mass transfer between a liquid and a gaseous stream, such as a packed bed, a wetted film, a spray tower, etc.
While the functional integration of various fuel cell power plant components has increased efficiency of the power plant, the structure of the housing integrating the direct mass and heat transfer device with the degasifier and coolant accumulator has given rise to an additional difficulty. In particular, to effect appropriate decontamination of the coolant fluid, the housing includes an upper portion or splash plate that causes vigorous splashing and turbulence of the cooling fluid within the housing prior to the cooling fluid passing through a degasifier material, such as a packed bed. Additionally, the plant exhaust also passes through the degasifier with the cooling fluid before passing over an exhaust surface of the mass transfer medium. Consequently a substantial portion of the cooling fluid may thereby pass as entrained liquid with the plant exhaust stream into and through the mass transfer device, resulting in an unacceptable loss of cooling fluid from the power plant.
Another problem associated with use of fuel cell power plants in transportation vehicles is contamination of power plant components by fine dust particles common to roadways upon which vehicles travel. Efficient fuel cell power plant design limits parasite power from the fuel cell to pump the process oxidant stream through fine particle filters. However, use of the aforesaid direct mass and heat transfer system to directly transfer heat and water into an ambient pressure process oxidant stream raises a risk of fine dust contamination of the transfer medium, fuel cell electrolyte and other plant components. A further difficulty of fuel cell power plants in transportation vehicles is limiting sound of the plant to an acceptable level. In known fuel cell power plants, such as that described in the aforesaid, commonly owned, U.S. Patent to Fuller et al., a blower is located on an oxidant inlet that directs the process oxidant stream into the fuel cell. Locating the blower between the direct mass and heat transfer device and the fuel cell enables the mass and heat transfer device to limit the noise of the blower, however a greater noise reduction is desirable.
Accordingly, there is a need for an improvement to known fuel cell power plants that provides for enhanced water retention in a functionally integrated direct mass and heat transfer device; that provides for enhanced filtration of fine dust particles within the process oxidant stream entering the fuel cell; and, that provides for further sound reduction of a blower that blows the oxidant stream into the fuel cell.
The invention is an interdigitated enthalpy exchange device for a fuel cell power plant that generates electrical energy from process oxidant and reducing fluid reactant streams. The power plant includes at least one fuel cell for producing electricity from the process oxidant and reducing fluid streams, and a direct mass and heat transfer device secured in fluid communication with both an oxidant inlet line that directs the process exhaust stream into the fuel cell and with a plant exhaust passage that directs a plant exhaust stream out of the power plant. The direct mass and heat transfer device includes a structure that secures the interdigitated fine pore enthalpy exchange device in mass transfer relationship between the process oxidant and plant exhaust streams.
The interdigitated enthalpy exchange device includes a fine pore process oxidant body having a process oxidant barrier surface opposed to an interdigitated process oxidant surface including at least one process oxidant entry channel defined in the interdigitated process oxidant surface that is in fluid communication with the process oxidant stream entering the fuel cell and that is discontinuous with at least one process oxidant exit channel defined within the interdigitated process oxidant surface so that a process oxidant barrier wall is defined between the process oxidant entry and exit channels. A process oxidant large pore media is secured adjacent the interdigitated process oxidant surface and overlies the entry and exit channels so that the oxidant stream passes from the process oxidant entry channel through the process oxidant large pore media and into the process oxidant exit channel that is in fluid communication with an oxidant inlet line extension directing the oxidant stream into the fuel cell. The interdigitated enthalpy exchange device also includes a fine pore exhaust body having an exhaust barrier surface opposed to an interdigitated exhaust surface including at least one exhaust entry channel defined within the exhaust surface that is in fluid communication with the exhaust passage and that is discontinuous with at least one exhaust exit channel so that an exhaust barrier wall is defined between the exhaust entry and exit channels. An exhaust large pore media is secured adjacent the exhaust surface and overlies the exhaust entry and exit channels so that the process exhaust stream passes from the exhaust entry channel through the exhaust large pore media into the exhaust exit channel that is in fluid communication with a plant exhaust vent. The exhaust barrier surface of the fine pore exhaust body is secured adjacent the process oxidant large pore media so that mass and heat, such as heated water vapor and entrained liquid water, within the plant exhaust stream passing through the exhaust entry and exit channels passes through the fine pore exhaust body and its exhaust barrier surface and into the process oxidant large pore media to enter the process oxidant stream passing through the process oxidant entry channel, the process oxidant large pore media and the process oxidant exit channel to humidify and heat the process oxidant stream entering the fuel cell.
The process oxidant and exhaust fine pore bodies include a support matrix that defines pores and a liquid transfer medium that fills the pores creating a gas barrier. In a preferred embodiment, the support matrix of the process oxidant and exhaust bodies defines pores having a pore-size range of less than 20 microns; the matrix is hydrophilic so that it is capable of being wetted by the liquid transfer medium resulting in a bubble pressure that is greater than 0.2 pounds per square inch (xe2x80x9cp.s.i.xe2x80x9d); and, the matrix is chemically stable in the presence of the liquid transfer medium.
Exemplary support matrixes include rigid, porous, graphite layers; rigid, porous, graphite-polymer layers; rigid, inorganic-fiber thermoset polymer layers; glass fiber layers; synthetic-fiber filter papers treated to be wettable; porous metal layers; perforated metal layers wherein such perforations may include particulate matter secured within the perforations defining an acceptable fine pore-size range; and a plurality of differing layers of those support matrixes. Preferably the support matrix has a high thermal conductivity to help transfer heat from the exhaust stream to an ambient air process oxidant stream to thereby minimize freezing within the oxidant inlet when operating at very low ambient temperatures. The liquid transfer medium may include water, and organic antifreeze water solutions, wherein the liquid transfer medium is capable of sorbing a fluid substance consisting of polar molecules such as water from a fluid stream consisting of polar and non-polar molecules.
The process oxidant and exhaust large pore media may be a carbon paper, such as a porous carbon-carbon fibrous composite having approximately sixty-five to seventy-five percent porosity and a mean pore diameter of greater than 20 microns, and the carbon paper may be wetproofed.
In an additional preferred embodiment, the direct mass and heat transfer device may be integrated with a power plant coolant stream to de-gasify the coolant stream and with a combusted anode exhaust stream to transfer moisture and heat from the combusted anode exhaust stream to the process oxidant stream entering the fuel cell. The coolant stream may also be used to replenish the liquid transfer medium of the interdigitated enthalpy exchange device.
In operation of the interdigitated enthalpy exchange device for a fuel cell power plant, as the plant exhaust stream passes through the exhaust entry channel, exhaust large pore media and exhaust exit channel, moisture and heat pass from the plant exhaust stream through a liquid transfer medium held within the fine pore exhaust body and its exhaust barrier wall into the process oxidant stream passing though the process oxidant entry channel, the process oxidant large pore media and the process oxidant exit channel. Because the exhaust entry and exit channels are discontinuous and separated by the exhaust barrier wall, most liquid moisture within the plant exhaust stream remains within the exhaust entry channel and will not pass through the exhaust barrier wall defined in the fine pore exhaust body. Thus, the liquid water will not pass with the gaseous process exhaust stream passing into the exhaust exit channels and plant exhaust vent to leave the plant. Additionally, the process oxidant entry and exhaust channels in the interdigitated process oxidant surface also serve to filter fine dust particles within the process oxidant stream, and to suppress noise of a blower located between the direct mass and heat transfer device and the fuel cell that could pass along the process oxidant stream and out of the plant.
Accordingly, it is a general object of the present invention to provide an interdigitated enthalpy exchange device for a fuel cell power plant that overcomes deficiencies of prior art fuel cell power plants.
It is a more specific object to provide an interdigitated enthalpy exchange device for a fuel cell power plant that transfers heat and water vapor from a plant exhaust stream leaving the plant directly into a process oxidant stream entering the plant.
It is yet another object to provide an interdigitated enthalpy exchange device for a fuel cell power plant that restricts movement of liquid water out of the plant.
It is still a further object to provide an interdigitated enthalpy exchange device for a fuel cell power plant that suppresses noise of the power plant.
It is another specific object to provide an interdigitated enthalpy exchange device for a fuel cell power plant that filters fine dust particles within the process oxidant stream entering a fuel cell of the power plant.
It is a further specific object to provide an interdigitated enthalpy exchange device for a fuel cell power plant that is integrated with a cooling fluid degasifier and accumulator.
These and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.